1. Field of the Invention
The present invention pertains to apparatus for enabling a floating vessel or a fixed platform located in an ice covered body of water to resist the lateral forces exerted on it by a layer of ice to either remain on station or to move through the ice water.
2. Description of the Prior Art
When drilling or producing oil offshore, it is essential that the structure from which such operations are conducted stays in position over the wellbore. In the more northern and southern climates of the world, winter brings on freezing weather causing large ice layers to form on the bodies of water. Due to winds, tides and currents, these ice layers tend to move. Sometimes the forces present in these moving ice layers are capable of overturning fixed structures and oftentimes are sufficient to force floating vessels off location. To avoid these accidents and interruptions, it is necessary somehow to decrease the magnitude of the lateral forces of the ice layer acting on the offshore structures. Other than designing a fixed structure to withstand the expected ice layer forces, the apparent solution is to break and clear the ice surrounding the structure.
For fixed structures, proposed methods of breaking and clearing the ice have included mounting cones on the structure's support members to force the ice sheet to bend and fail as it moves against the cone's inclined surface, melting the ice layer around the structure, and milling the ice layer with ice cutters into relatively small pieces. Many of these proposed methods could be adapted for use by a floating vessel, with the ice milling method being advanced as the most promising. Yet, energy requirements for ice milling are expected to be extremely high and the technical and economic feasibility of this approach is by no means certain. Therefore a need exists for a practical system that will economically allow offshore structures to resist the lateral forces of moving layers of ice.
Systems for chipping ice are described in U.S. Pat. No. 3,768,428, and U.S. Pat. No. 3,888,544. A system for actually cutting ice is described in U.S. Pat. No. 3,921,560.
The device of U.S. Pat. No. 3,921,560 for cutting ice is located on the bow of a ship whereby the cutting member is a rotary, horizontally-mounted, helicoidal member.
Furthermore, it is known that screw devices of certain design can be used for supplying the propulsion force for a vessel whereby the screw is located below the surface of the water. This is illustrated by Hoke U.S. Pat. No. 2,806,441.
In addition, Ellis U.S. Pat. No. 1,482,511 shows a plurality of endless chains with ice scraping and breaking teeth mounted on the bow of the boat.
It is mentioned in U.S. Pat. No. 3,667,416 that the art is familiar with "Ships having a stem with a slope from the front to the rear, which permits the bow of the ship, again by the action of the propulsive force, to mount on the ice which is broken by the effect of the weight of the ship." Nothing is said, however, with respect to assisting the ice breaking bow of the ship to rise up (ride) on the ice by using dragging or pulling means in order to utilize the weight of the ship more effectively in combination with the propulsive force of the ship. Nor is anything said with respect to utilizing dragging or pulling means, without other propulsive force, to urge the ice-breaking bow of a ship to mount upon the ice layer.
Canadian Pat. No. 1,011,605 published 6/7/77 (too late to be a reference as of its publication date) discloses an ice breaking hull with support means mounted on a pontoon-shaped forecastle which means bear on unbroken ice.
Also, no references are known which are relevant to the embodiment utilizing a vertically oriented screw means to protect a vertical member at an approximately 90.degree. angle to an ice layer.